Write the effect of concentration of $CO_2$ on $C_3$ and $C_4$ plants.

(N/A) The concentration of $CO_2$ is a major limiting factor for photosynthesis.
$1$. $C_3$ plants: These plants show an increase in the rate of photosynthesis with an increase in $CO_2$ concentration up to $450 \ \mu L/L$. Since $RuBisCO$ has a lower affinity for $CO_2$ and also acts as an oxygenase,higher $CO_2$ levels suppress photorespiration,thereby increasing productivity.
$2$. $C_4$ plants: These plants show saturation at much lower levels of $CO_2$ (around $360 \ \mu L/L$). Because $C_4$ plants have a $CO_2$ concentrating mechanism (the $C_4$ cycle) that ensures $RuBisCO$ operates in a high $CO_2$ environment,they do not show significant increases in photosynthesis at higher $CO_2$ concentrations compared to $C_3$ plants.